A motor is driven by applying a line-to-line voltage to the motor so as to pass a current through an armature of the motor. On the other hand, when the motor is rotated, a counter electromotive voltage occurs. When the applied voltage is higher than the counter electromotive voltage, a current flows so that the motor can be driven. When the number of rotations of the motor increases, the counter electromotive voltage increases. Since there is an upper limit of the voltage to be applied, there finally occurs a voltage saturation in the armature of the motor and thus the current cannot flow any more. Thereby, it becomes impossible to drive the motor because of lack of a torque.
In order to obtain a larger torque, it is necessary to increase the voltage to be applied. However, a large equipment is necessary therefor, which is expensive. Alternatively, it can be considered that capacities of the motor and an inverter that drives the motor are increased. However, this solution also needs a high cost and is undesirable. Thus, costless methods of preventing occurrence of the voltage saturation in order to generate a larger torque have been recently proposed.
As a method of preventing occurrence of the voltage saturation in an armature of a motor, there has been proposed a method of controlling a current which can achieve a higher-speed higher-output performance, by passing a d-axis current through the armature in which a d-axis direction corresponds to a direction of a magnetic field in the motor, so as to reduce a counter electromotive voltage that occurs when the motor is rotated, whereby a q-axis current for generating a torque can be increased (see, e.g., JP9-84400A).
In addition, as a method of achieving a higher output performance, there has been proposed a control method which can increase an effective voltage as compared with a general case where a sinusoidal phase voltage is supplied to each phase, by making a line-to-line voltage waveform into a rectangular or pseudo-rectangular waveform (see, e.g., JP3939481B).
Although the control method disclosed in JP9-84400A is stable, it cannot be said that the capacity of the motor is drawn out as much as possible, when a circuit of an inverter for driving the motor is sufficiently enough. This is because a reactive current is controlled to be passed at a set speed, which is near to a speed at which a voltage saturation occurs. On the other hand, since the control method disclosed in JP3939481B makes a line-to-line voltage waveform into a rectangular or pseudo-rectangular waveform, the capacity of the motor can be drawn out as much as possible when the circuit of the inverter for driving the motor is sufficiently enough. However, this method lacks a stability, and there is a possibility that the current control becomes unstable in the worst case.